The present invention is generally directed to processes for the preparation of branched or dendrimeric homopolymers and copolymers. More specifically, the present invention relates to improved polymerization processes which provide homopolymer and copolymer resin products which possess narrow polydispersity properties and which polymerization processes proceed with high monomer to polymer conversion. In particular, this invention relates to stable free radical mediated or pseudoliving polymerization processes which yield branched homopolymers and copolymers having number average molecular weights (M.sub.n) above about 100 to about 200,000 and having a polydispersity ratio of the weight average molecular weight (M.sub.w) to the number average molecular weight (M.sub.n) of from about 1.0 to about 2.0 and which processes accomplished in accordance with the present invention provide numerous operational and economic advantages.
The present invention provides in embodiments a pseudoliving polymerization process that enables the synthesis of narrow polydispersity branched homopolymer and copolymer resins from a variety of free radical reactive monomers. The process can, in embodiments, use known free radical initiators in combination with nitroxide or non-nitroxide type stable free radical agent compounds and free radical reactive monomers to afford narrow polydispersity dendrimeric thermoplastic resins or elastomers. In other the embodiments, the present invention provides processes for preparing highly branched linear and star type dendrimer molecules by sequentially conducting an alternating series of stable free radical mediated polymer chain extension or growth reactions, at elevated temperatures, wherein monomer is added substantially uniformly, with respect to molecular weight of added monomers, to all of the propagating arms or branches; and chain branching reactions wherein a free radical reactive branching agent is reacted at, for example, lower temperatures with the propagating polymer chains thereby introducing additional branch points in the dendrimer molecule. The sequence can be repeated numerous times and to the extent that the preceding product polymer containing stable free radical terminal functional groups is capable of being thermally activated or otherwise further reacted to provide further chain branching or chain extension propagation.
Many polymerization processes used for the synthesis of narrow polydispersity resins, such as anionic, cationic, and group transfer polymerization processes, are severely limited by the need for anhydrous reaction conditions and monomers which do not contain protic or reactive functional groups, for example, hydroxy (OH) carboxy (CO.sub.2 H), amino (NH), and the like. As a consequence, these processes are not readily applicable to the polymerization of functionalized monomers since these monomer materials tend to be hydroscopic and any associated water may readily destroy the polymerization initiator component, for example, the hydrolysis or protonation of organolithium reagents by the monomer in anionic polymerization processes, or in other ways cause the polymerization to fail entirely or to be industrially inefficient.
It is generally accepted that known anionic and cationic polymerization processes used for the preparation of branched or dendrimeric narrow polydispersity resins, block and multiblock polymers are not believed possible in aqueous or protic solvent containing polymerization media, or the aforementioned protonic or reactive functional groups, reference commonly assigned U.S. Pat. No. 5,312,704.
Of the known polymerization processes a preferred way to prepare branched or dendrimer polymers or copolymers having a narrow molecular weight distribution or polydispersity is by anionic processes. The use and availability of branched or star type resins having narrow polydispersities in industrial applications is limited because anionic polymerization processes must be performed in the absence of atmospheric oxygen and moisture, require difficult to handle and hazardous initiator reagents, and consequently such polymerization processes are generally limited to small batch reactors. In addition, the monomers and solvents that are used must be of high purity and anhydrous thereby rendering the anionic process more expensive than alternatives which do not have these requirements. Thus, anionic polymerization processes for the preparation of branched or star type polymers are difficult and costly. It is desirable to have free radical polymerization process that provides narrow molecular weight distribution branched or star type polymers and resins that overcomes the shortcomings and disadvantages of the aforementioned related anionic branching polymerization processes.
Similarly, group transfer polymerization (GTP) processes have limitations and disadvantages, such as anhydrous reaction conditions and expensive reagents, which disadvantage GTP processes particularly for large scale industrial preparation of branched or star polymers.
Free radical polymerization processes are generally chemically less sensitive than anionic processes to impurities in the monomers or solvents typically used and are substantially or completely insensitive to water. There has been a long felt need for an economical free radical polymerization processes which are suitable for preparing narrow polydispersity branched or star type resins in the presence of water.
Conventional free radical polymerization processes that are used to polymerize monomers in general, and functionalized monomers in particular, inherently give broad polydispersity resin products or require that sophisticated processing conditions and materials handling protocols be employed.
Star polymers or dendrimers can be constructed either with a rod-like tertiary structure or a spherical tertiary structure. The branching associated with these systems provide a number of unusual properties. For example, in contrast to linear polymers, the viscosity of spherical star polymers decrease as the molecular weight increases. Furthermore, compounding hydrocarbon dendrimers in a variety of media imparts increased strength to these materials.
For example, E. L. Hillier, U.S. Pat. No. 4,048,254, has described the improvement of thermoplastic resins by blending in polystyrene star polymers. T. E. Kiovsky, U.S. Pat. No. 4,077,893, has described the use of star polymers for improving the viscosity index of lubricating oils. Kiovsky has also described the use of these materials as a dispersant. W. R. Haaf et al, U.S. Pat. No. 4,373,055, has shown that the impact strength of polyphenylene ether resins can be improved by the addition of hydrocarbon star polymers while other similar applications have also been described by M. H. Lehr, U.S. Pat. No. 4,181,644, W. P. Gergen et al., U.S. Pat. No. 4,242,470, and A. Aoki et al., U.S. Pat. No. 4,304881. Other illustrative example of star polymers and their properties are disclosed in H. Eschway, M. L. Hallensleben, and W. Burchard, Makromolek. Chem., 173, 235 (1973), and D. A. Tomalia, A. M. Naylor, and W. A. Goddard III, in Angew. Chem. Int. Ed. Engl,. 29, 138 (1990).
In commonly owned and assigned U.S. Pat. Nos. 5,098,475 and 5,120,361, there are illustrated inks with dendrimers, and more specifically, in U.S. Pat. No. 5,098,475, the disclosure of which is totally incorporated herein by reference, there is disclosed an ink composition which comprises an aqueous liquid vehicle and a colored dendrimer, or dendricolorant obtained by attaching a reactive dye or dyes to commercially available amino terminated dendrimers, that is for example a dendrimer having an amine group such as NH.sub.2 attached to the end of the arm farthest removed in distance from the core, which dendrimers can be of the first, second, third, or n-th generation, wherein n is a number of preferably less than 9, and more specifically is a number of from about 1 to about 8, and wherein the attachment reaction can be accomplished at room temperature in water.
Branched copolymers prepared by conventional free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about four. One reason is that most free radical initiators selected have half lives that are relatively long, from several minutes to many hours, and thus the polymeric chains are not all initiated at the same time and which initiators provide growing chains of various lengths at any time during the polymerization process. Another reason is that the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating and polydispersity broadening reaction processes. In doing so, chains of varying lengths are terminated at different times during the reaction process which results in branched resins comprised of polymeric chains which vary widely in length from very small to very large and thus have broad polydispersities. If a free radical polymerization process is to be enabled for producing branched, dendrimeric, or star polymers with narrow molecular weight distributions, then all polymer chains must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided or eliminated.
Practitioners in the art have long sought an inexpensive, efficient and environmentally efficacious means for producing branched polymers having operator controllable or selectable molecular weight and branching properties, and further, processes which selectively afford a wide variety of different polymer product types and have narrow molecular weight distribution properties.
In the aforementioned U.S. Pat. No. 5,322,912 there is disclosed free radical polymerization processes for the preparation of a thermoplastic resin or resins comprising: heating from about 100 to about 160.degree. C. a mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. A broad spectrum of free radical reactive monomers are suitable for use in the highly versatile polymerization process. While a variety of homopolymers and copolymers, including block and multiblock copolymers, could be prepared with high conversions and narrow polydispersities, no mention was made or suggested to include a branching agent in the polymerization process or to prepare branched, star, or dendrimeric polymer resins.
The following patents are of interest to the background of the present invention, the disclosures of which are incorporated by reference herein in their entirety:
In European Patent Publication 349,270 B1, filed Jun. 6, 1988 (US), is disclosed a pressure-sensitive adhesive composition characterized by comprising: a block copolymer represented by the general formula I(BAT).sub.n wherein I represents the free radical initiator portion of an iniferter of the formula I(T).sub.n ; T represents the termination portion of said iniferter; n is an integer of at least 2; and B represents an elastic acrylic polymer block having a glass transition temperature of at least 30.degree. C. and an A-block is formed of a monomer selected from the group consisting of methyl methacrylate, polystyrylethyl methacrylate, macromer, methyl methacrylate macromer, acrylic acid, acrylonitrile, isobornyl methacrylate, N-vinyl pyrrolidone, and mixtures thereof, the weight ratio of said B-block to said A-block in said block copolymer being from 95:5 to 50:50; and 0 to 150 parts by weight tackifier per 100 parts block copolymer. Also disclosed is a method of making the pressure sensitive adhesive which relies upon mixing and exposing an iniferter of the formula I(T).sub.n to an energy source in the presence of a sequence of monomer charges.
U.S. Pat. No. 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator having the formula (in part) =N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100 degrees C, and use multiple stages.
European Patent Publication No. 0135280 corresponding to European Patent Application No. EP 84 304,756 is the European Patent Office equivalent of the aforementioned U.S. Pat. No. 4,581,429.
In Polymer Preprints, 35 (1), 778 (1994), Matyjaszewski et al., is disclosed thermal polymerizations of styrene monomers in the presence of stable radicals and inhibitors, but without a free radical initiator present, such as peroxide or azo compounds.
In the Journal of the American Chemical Society, 1994, 116, p. 11185-11186, is disclosed free radical polymerization processes for the preparation of narrow polydispersity polymers, such as polystyrene, and block copolymers, such as poly(styrene-b-acetoxymethyl styrene), using a free radical initiator, such as benzoyl peroxide, a stable free radical compound such as TEMPO, and a monomer, in accordance with the prior teachings of Georges et al., Macromolecules, 1993, p. 26, 2987, which prior teaching is based on the aforementioned commonly assigned U.S. Pat. No. 5,322,912.
In U.S. Pat. No. 5,268,437, to Holy, issued Dec. 7, 1993, discloses a high temperature aqueous processes for the polymerization of monoethylenically unsaturated carboxylic monomer to produce low molecular weight, water-soluble polymer products useful as detergent additives, scale inhibitors, dispersants and crystal growth modifies. Suitable monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, crotonic acid, and itaconic acid. The reactions are run at temperatures ranging from about 130 to 240.degree. C., preferably from about 140 to about 230.degree. C., with polydispersities less than 2.5. The process can be continuous, semicontinuous, or batch.
In U.S. Pat. No. 4,546,160, to Brand et al., issued Oct. 8, 1985, is disclosed a process to continuously bulk polymerize acrylic monomers to prepare low molecular weight, uniform polymers employing minor amounts of initiator and, optionally solvents, at short residence times and moderate reaction temperatures to provide high yields of a product with polydispersities less than 3, suitable for high solids applications.
U.S. Pat. No. 5,059,657 to Druliner et al., issued Oct. 22, 1991, discloses a polymerization process for acrylic and maleimide monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
U.S. Pat. No. 4,736,004 to Scherer, Jr. et al., issued Apr. 5, 1988, discloses novel persistent perfluorinated free radicals which, upon thermal decomposition, yield free radical species which can be used to polymerize polymerizable monomers containing ethylenic unsaturation.
U.S. Pat. No. 3,600,169 to Lawton, issued Aug. 17, 1971, discloses an electrostatic light sensitive reproduction sheet employing a composition comprising in an insulating resinous binder an organic stable free radical and a precursor sensitive to light to be converted to transient free radicals that are reactive with the stable free radical and to change the conductance of the sheet so that an electrostatic image can be formed. Also disclosed is an extensive listing of stable free radical compounds.
Other references cited in an international search report for the aforementioned commonly owned U.S. Pat. No. 5,322,912 are: J. Am. Chem. Soc., 1983, 5706-5708; Macromol., 1987, 1473-1488; Macromol., 1991, 6572-6577; U.S. Pat. No. 4,628,019 to Suematsu et al., issued Aug. 10, 1986; U.S. Pat. No. 3,947,078 to Crystal, issued Aug. 10, 1976; and U.S. Pat. No. 3,965,021 to Clemens et al., issued Jun. 22, 1976. The cited references disclose alternative means, as discussed above, such as anionic, group transfer, and the like, for preparing polymer resins, and in some instances branched or star type polymers, with narrow polydispersity properties, but which alternative means do not provide the aforementioned convenience and economic advantages of the present invention.
One known method of achieving control of polymer molecular weight is through the use of efficient chain transfer agents, but this approach has several drawbacks. This approach irreversibly incorporates the structure of the chain transfer agent into the polymer chain. This can be undesirable since that structure will have an increasing effect on the properties of the polymer as molecular weight decreases. Furthermore, the chain transfer agents commonly employed are mercaptans. These materials are expensive and have objectionable odors associated with their presence. Other common chain transfer agents are hypophosphites, bisulfites, halogenated hydrocarbons such as carbon tetrabromide, and alcohols. These also add to the cost of the process, introduce undesired functionally or properties to the polymer, can introduce salts into the product, and may necessitate an additional product separation step to remove the chain transfer agent from the reaction mixture.
Another way of lowering the molecular weight of the polymer product is by increasing the amount of free radical initiator. This approach adds considerably to the cost of production and may result in polymer chain degradation, crosslinking, and high levels of unreacted initiator remaining in the product. In addition, high levels of initiator may also result in high levels of salt by-products in the polymer mixture which can be detrimental to polymer performance in many applications. The same is true for chain stopping agents such as sodium metabisulfite.
High levels of metal ions together with high levels of free radical initiator have also been tried as means for controlling molecular weight. This method is taught in U.S. Pat. No. 4,314,044 where the ratio of initiator to metal ion is from about 10:1 to about 150:1 and the initiator is present from about 0.5 to about 35 percent based on the total weight of the monomers. Such an approach is unsuitable for some products, such as water treatment polymers, which can not tolerate metal ion contaminants in the polymer product. In addition, the product is usually discolored due to the presence of the metal ions. Thus, polymerization processes which produce product polymers containing residual non-polymeric materials may be significantly negatively compromised with respect to, for example, appearance, performance and toxicity properties. Polymeric processes which create or contain non-polymeric residual materials are preferably avoided.
The following references are also of interest: U.S. Pat. Nos. 3,682,875; 3,879,360; 3,954,722; 4,201,848; 4,542,182; 4,581,429; 4,777,230; 5,059,657; 5,173,551; 5,191,008; 5,191,009; 5,194,496; 5,216,096; and 5,247,024.
In free radical polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling both the polydispersity and modality of the polymers produced. These polymerization processes produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities or broad molecular weight distribution (M.sub.w /M.sub.n) and in some instances low conversion. Further, polymerization processes of the prior art, in particular free radical processes, are prone to generating excessive quantities of heat since the polymerization reaction is exothermic. As the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This is particularly the situation for reactions with high concentrations of soluble monomer, for example greater than 30 to 50 percent by weight soluble monomer, which are conducted in large scale reactors with limited surface area and limited heat dissipation capacity. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin, particularly for highly concentrated reactions.
Other disadvantages associated with the prior art methods for preparing branched or star type dendrimeric polymeric materials is the they typically provide products with highly variable branch length, variable branch molecular weight and polydispersities, and variable particle size, for example.
These and other disadvantages are avoided, or minimized with the branching polymerization processes of the present invention.
Thus, there remains a need for branching polymerization processes for the preparation of branched or dendrimeric, narrow polydispersity, polymeric resins by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, uniform branch or arm length and number, hardness, low gel content, processibility, clarity, high gloss durability, and the like, while avoiding the problems of non-uniform branch length or composition, gel formation, exotherms, volume limited and multi-stage reaction systems, complex purification, encumbered or compromised performance properties due to undesired residuals, broad polydispersity properties of the polymer resin products, and the like, associated with prior art free radical polymerization methodologies.
There also remains a need for polymerization processes which enable the preparation of narrow polydispersity branched or dendrimeric compounds and polymers with high molecular economy and efficiency, and by providing alternative synthetic pathways, such as linear, convergent, and de novo routes, and which compounds and polymerization processes overcome the aforementioned limitations and problems.
The polymerization processes and the resultant branched, dendrimeric and star compounds, thermoplastic resins, and elastomer products of the present invention are useful in many applications, for example, as a variety of specialty applications including toner and liquid immersion development ink resins or ink additives used for electrophotographic imaging processes, or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable, for example, in thermoplastic films, electrophotographic marking materials such as toners and toner additives, and aqueous or organic solvent borne coating technologies.